Our research objectives are to 1) increase our understanding of genetic recombination in cultured mammalian cells and 2) to use homologous recombination as a genetic tool for analyzing mammalian development and disease. Towards the first objective we will study recombination in cultured mammalian cells between cointroduced DNA sequences and between newly introduced DNA sequences and homologous chromosomal sequences. Toward the second objective we will attempt to develop the technology for mutating genes in the mouse by targeting DNA sequences to specific sites in the mouse genome. In vitro mutagenesis will be used to generate the desired mutation in a cloned DNA sequence, and gene-targeting will be used to introduce the mutation into the genome. We propose to use three test systems for these experiments, the correction of mutant neomycin resistant genes integrated into the host genome, the mutation of the endogenous adenine phosphoribosyl transferase gene and the mutation of the endogenous hypoxanthine phosphoribosyl transferase gene. If successful, this technology will provide a new approach for genetic analysis of a complex organism such as the mouse as well as provide a means for generating animal models for human diseases.